Sulfide dust explosions have occurred in underground mines in various parts of the world, particularly in mines where the ore body contains massive sulfide deposits that have sulfur contents as high as 50% or more. Although the sulfide concentration is deemed to be the major contributor to the explosion incident, other chemical, geologic or physical factors also may contribute to the propensity of a sulfide ore body to experience afterblast dust explosions.
A possible explanation for the dust explosion is that the flame generated by the detonating blasting agent ignites the sulfide dust generated by the detonation or blast itself (or the dust could be present from prior blasting or other mining activities). The resulting dust explosion can inflict considerable damage to a mine and present an injury potential to personnel within the mine. These explosions also can produce large quantities of sulfur dioxide and other noxious gasses that can permeate a mine's atmosphere for hours. Thus dust explosions result in substantial productivity losses in mining operations.
Attempts to control afterblast dust explosions have centered on: the type of explosives used, such as ANFO, packaged products, bulk products, etc.; reducing the incendivity characteristics of the explosives through formulation variations; the design and setup of the blast, including the use of stemming materials of various kinds; other precautions taken at the blast face to reduce or cool explosive flash, such as misting, hanging lime bags, etc.; and general cleanup or wetting of any dust in the drift and at the face. These approaches, although undoubtedly helpful, have been insufficient in the more difficult ore types where afterblast sulfide dust explosions occur with nearly every blast.
Emulsion blasting agents are well-known in the art, and in general, have superior properties to other commonly used blasting agents, such as ANFO or packaged blasting agents, in minimizing the potentiality of afterblast sulfide dust explosions. The use of an emulsion blasting agent by itself, however, is not sufficient to prevent afterblast sulfide dust explosions in all instances, and importantly it has been discovered in the present invention that the presence of a chemical inhibitor, preferably urea, functions as stated previously to suppress the rapid, energetic reaction of afterblast residual nitrates or NO.sub.x from reaction with sulfide dusts. Thus a critical element of the present invention is to add a chemical inhibitor to the emulsion blasting agent.
Urea has been used or suggested for use in water-bearing blasting agents of the emulsion or water-gel type and in ANFO blasting agents. However, specific uses in emulsions have been limited and have generally focused on obtaining specific effects. For example, U.S. Pat. No. 5,608,185 discloses the use of urea in the oxidizer salt solution phase of an emulsion blasting agent for purposes of reducing the formation of NO.sub.x fumes resulting from the detonation of the emulsion blasting agent. U.S. Pat. No. 5,159,153 discloses the use of urea in the oxidizer salt solution phase of an emulsion blasting agent for purposes of stabilizing the blasting agent against preblast thermal degradation in the presence of reactive sulfide and pyrite ores. U.S. Pat. No. 4,338,146 discloses the use of urea as an additive in a cap-sensitive emulsion explosive in an amount of less than 5% by weight to lower the crystallization point. U.S. Pat. No. 4,500,369 discloses the use of urea in an emulsion blasting agent to lower its crystallization temperature. U.S. Pat. No. 3,708,356 discloses the use of urea to stabilize ANFO against preblast reaction with pyrite ores. These patents do not suggest, however, the use of urea for the purposes described herein. As previously indicated, urea is not a "normal" ingredient in present commercial repumpable emulsions. In already fuel rich emulsions, it is generally considered to be undesirable to have water soluble fuels that, in effect, further limit the percentage of continuous phase (fuel phase) that can be used while still maintaining reasonable oxygen balance.